Cancer progression and metastasis require changes in cell polarity that drives changes in cellular adhesion and motility. Adhesion and motility are regulated by the polarized actin cytoskeleton. Our long-term goal is to identify the mechanisms that polarize actin during healthy growth and during disease. We have used the model organism C. elegans to study cell movements during embryonic development. Using knockout mutants we have shown that loss of regulators of actin nucleation including the GTPase CED-10/Rac1, any component of the actin nucleating Arp2/3 complex, or of its activator, the WAVE/SCAR complex, results in the same phenotype: failure in embryonic cell migrations, morphogenesis and altered epithelial polarity. Mutations in the conserved WAVE/SCAR complex are associated with cancers including aggressive metastatic cancers. For example, WAVE2 is misexpressed in malignant lung cancers and metastatic colorectal cancers (Semba et al. 2006; Iwaya et al. 2007). However, how the actin cytoskeleton is regulated during normal growth or misregulated during metastasis is not well understood. We have recently identified extracellular signals that regulate Rac1/CED-10 and WAVE/SCAR during embryonic cell migrations. With these upstream signals in hand we want to address the following hypothesis for how outside signals polarize F-actin: Objective/Hypothesis: We hypothesize that distinct signals at the plasma membrane activate the WAVE/SCAR complex to promote cell polarization, and that the extracellular receptors we have identified play an important role in both signaling between cells, and in transmitting signals intracellularly to polarize cellular F-actin. Specific Aims: (1) To tes the model that ECM (extra cellular matrix) receptors support epithelial cell migrations by regulating WAVE at the apical junction. (2) To determine if signaling between tissues organizes F-actin in epidermal cells. (3) To test the model that ECM receptors act through the regulation of specific Rac GAPs. Study Design: In Aim 1 we use our in vivo system to determine both how WAVE is recruited to apical junction and what role it plays there to better understand how WAVE/SCAR promotes cell polarity. In Aim 2 we create an in vivo model for testing how tissues detect the polarized state of F-actin in neighboring tissues. In Aim 3 we use genetics and biochemistry to identify the GAP proteins that regulate Rac/CED-10 during embryonic morphogenesis. Clinical relevance: The human homolog of one of the genes we study in C. elegans, WAVE3, is considered a biomarker for high grade, triple negative breast cancer (Kulkarni et al., 2012) and is associated with invasive prostate and colon cancers (Fernando et al., 2010; Zhang et al., 2012). Understanding the molecules that regulate actin dynamics through the WAVE/SCAR complex during cell migrations will not only enlighten our understanding of normal development but could suggest new biomarkers for altered actin regulation in human disease.